1. Field of the Invention
The problem of building security is an important issue currently being addressed by developing technology. Automatic counting mechanisms for determining and/or controlling the number or identity of people passing through a particular entrance or exit have been around for some time. They vary from the simple automatic turnstile to swipe card access and radio-frequency tagging systems. A principal disadvantage of all these previous techniques is the low access speed. An automatic turnstile is particularly obstructive in requiring considerable effort to be made by moving personnel. None of the systems can be operated with a default unlocked door making a locking/unlocking mechanism unavoidable. In swipe card systems such door mechanisms are prone to failure. Radio-frequency tagging doesn""t detect untagged intruders and so cannot be used to maintain a default unlocked system.
There is a perceived need for a detection system capable of monitoring personnel movement which provides for a faster throughput of traffic.
It is an object of this invention to provide a detection system capable of providing non-intrusive monitoring of personnel movement through default unlocked entry and exit points.
The present invention provides a detection system including a light source arranged to project light onto an obstructing surface and imaging means arranged to selectively image the intersection of the projected light with the obstructing surface characterised in that the imaging means comprises at least one camera sensitive to light emitted from said light source and the imaging means and light source are mutually arranged such that the orientation of imaging means optical axis and direction of light projection from said source are parallel or intersect at a position on the opposite side of the obstructing surface to the imaging means and source of projected light.
The invention provides the advantage of non-intrusive detection. It exploits the effects of perspective on objects when viewed from a particular direction: an object interrupting such projected light can be readily detected as an outward movement of an imaged intersection in the field of view. It thus allows for the detection of object movement through the system without obstruction of the traffic flow.
The system is preferably arranged to monitor the presence or absence of bodies on a bounding surface intersecting the projected light. In this regard, the bounding surface forms the obstructing surface in the absence of a body; and the surface of the body forms the obstructing surface in the presence of a body. The imaged intersection then has different positions according to whether it arises from the bounding surface or the surface of the body and these positions are relatively displaced in accordance with the body surface""s remoteness from the bounding surface. This provides the advantage of simplicity in application of the invention to numerous situations. The intersection of the projected light with a surface can be imaged and that image monitored continuously. Movement of the image can be ascribed to an object on the surface passing through the projected light. The necessary movement may be provided by the object itself, motion of a level surface supporting the object or by movement of the light sheets and imaging means across a stationary surface and objects. Moreover, the objects need not be physically separable from the surface the invention can be used to map the structure of the surface itself.
The projected light is preferably in the form of a substantially planar sheet of light and the imaging means is arranged such that its optical axis is substantially parallel to and offset from the plane of the light sheet. This improves the detection capability of the system. The intersection with a level surface will thus be a bright line which will form a well defined image. Any object crossing this bright line anywhere along its length will cause a deflection in the image. Furthermore the narrowness of the line improves the capability of the system in its provision of accurate information regarding the height of an interrupting object.
The projected light may be in the form of two substantially parallel planar sheets of light disposed about the imaging means and the imaging means arranged such that its optical axis is substantially parallel to the planes of the light sheets. This provides the capability for determining direction of travel through the system by means of observing the sequence of light sheet disturbance.
The imaging means may be arranged to form an image of an intersection of the obstructing surface and a light sheet wherein the intersection is detectable as a line in the image. The system may be arranged to respond to a deflection of such an image line. This provides an improvement to utility. The system is capable of responding to the data received and the need for manual interpretation is reduced.
The system may be arranged to monitor the profile of bodies on a bounding surface intersecting a projected light sheet. In the presence of a body, the surface of the body forms the obstructing surface and the image line defines a deflection pattern characteristic of the profile of the body. This increases the information available to the detection system and renders it capable of more sophisticated responses.
The deflection pattern may comprise perpendicular displacements of image line components from their original positions in the line. The displacements (dshift1) measured at the image plane of the imaging means are described by the equation       d          shift      ⊥        =            fd      ⊥        ⁢                  h        body                              h          c                ⁢                  (                                    h              c                        -                          h              body                                )                    
where d1 is the perpendicular distance from the imaging means to the light sheet responsible for the line in the image, f is the focal length of the imaging means1 hc is the distance between the imaging means and the bounding surface and hbody is a parameter describing the body height at each point that it intersects the light sheet. This provides a straightforward means of deriving the profile of an interrupting object from the deflection pattern observed as the object passes through a light sheet.
The projected light may be directed transversely of a longitudinally extending transit zone, and projected from a bounding region in which the imaging means is located. This provides for complete coverage across the transit zone and the system is therefore capable of detecting any object moving along it. In one embodiment the transit zone may be a corridor, the obstructing surface is the corridor floor or the surface of a body and the bounding region is the corridor ceiling. This provides a detection system which is capable of monitoring personnel movement into and out of a designated room or area of a building.
The imaging means may be a single camera or a one-dimensional array of at least two cameras, the array alignment being substantially perpendicular to the component cameras"" optical axes and substantially parallel to the plane of the projected light. A single camera provides the advantage of cost reduction but an array will allow imaging along the length of the light sheets without demanding an extensive single camera field of view. Furthermore the array also provides an improvement in accurate counting. A single camera could find its view of one body obscured by a second, nearer body. This is particularly likely to occur if the second body is taller than the first. The effects of such obscuration are largely overcome by the use of a number of cameras as any one body will be imaged in at least one camera.
The light source may be at least one strip source extending longitudinally along one side of each light sheet. Alternatively, it may comprise an array of point sources for each sheet such that the array is located along one side of each light sheet and arranged to project light to the opposite side of the respective light sheet. Each point source may be associated with a cylindrical lens and thereby arranged to project light fanned within the plane of the respective light sheet. A strip source provides the advantage of security. Objects passing through the light sheet will disturb the light regardless of their position within the sheet. It also enables a profile of the object to be derived from the deflection pattern without interpolation. Fanning an array of collimated point sources will also enable complete transverse coverage, and this implementation is more cost effective. An unfanned collimated array will require more sources in order to achieve effective transverse coverage but it does reduce the problems of potential obscuration. In the fanned case, light may be blocked from illuminating a body by a second, closer body. Furthermore, using an unfanned collimated array simplifies both the computation associated with the invention and the implementation of its optics. With such a collimated array the projected light is imaged as a series of two-dimensional dots as opposed to a line. The locus of deflection of each dot is known and so only a limited number of pixels need to be searched in locating the deflection pattern. Moreover, a dot exhibits a two-dimensional intensity profile which increases the reliability of its detection.
The imaging means may be focused on a plane located within a height range 1.6 m to 2.0 m above the floor. This range is the average human height and so this feature enhances the focus of the deflection pattern in any implementation for which the system detects human traffic.
The imaging means is preferably connected to a data processing system responsive to the form and/or change in the image formed by the imaging means. This provides the system with a powerful capability to interpret and react to data received from the imaged intersections.
The data processing system may be arranged to respond with a count each time a deflection of the image line occurs. In embodiments employing two sheets of light, the data processing system is arranged to associate a deflection of one image line from a first position in the image to another position with a subsequent deflection of the other image line for the purpose of determining direction of travel of the body causing said deflections and further arranged to respond with a count on the occurrence of such a pair of deflections. In this way the system is capable of keeping an account of the number of personnel occupying a designated area guarded by the detection system of the invention. This is advantageous to the use of the system in secure or restricted access buildings.
A first deflection of one image line may be associated with a deflection of the other image line which occurs most immediately after the first deflection. Or, a deflection of one image line may be associated with a deflection of the other image line which occurs nearest to a predetermined time after the first deflection. These embodiments provide methods of association which are not particularly demanding of computing power in their application. Alternatively, the data processing system may be arranged to apply pattern matching techniques to match a deflection pattern at one image line with a deflection pattern at the other image line and thereby to associate said two deflections. This provides an increase in accuracy while monitoring two-directional flow of traffic. A body is not counted until its profile is registered first at one image line and then at the other.
The data processing system includes an image processor arranged to record, process and digitise the deflection pattern, a counting unit arranged to count the number of such deflection patterns occurring in each light sheet, an interpreter arranged to associate disturbances arising from the same body passing through both light sheets and a memory arranged to provide data to the interpreter. This provides a straightforward example of a data processing system suitable for implementation with the projected light detection apparatus which monitors the number of individuals passing through and direction of travel of each.
The data processing system may be arranged to compare a multiple-body deflection pattern occurring at an image line with stored deflection patterns, each stored deflection pattern being characteristic of a single causative body, and thereby to be capable of resolving the unknown deflection pattern into a number of overlapping single-body deflection patterns. This provides the advantage of increased flexibility by rendering the system adaptable to use in situations of high traffic flow.
The data processing system includes updatable storage means arranged to monitor the population within a designated area in accordance with the number of bodies passing through the detection system of the invention and the direction of travel deduced by association of deflections of different image lines, and wherein the number of single-body deflection patterns resolved from the multiple-body deflection patterns is equivalent to the number of bodies passing through a light sheet and therefore entering or leaving the designated area. This combines the advantages of constant monitoring of the population of a restricted area with the flexibility of adaptation for a high volume of traffic flow.
In a further embodiment, the detection system may be arranged to monitor the presence or absence of irregularities on a surface. In this embodiment, in the absence of an irregularity, the surface forms the obstructing surface; and in the presence of an irregularity, the surface of the irregularity forms the obstructing surface. The imaged intersection thus has different positions according to whether it arises from the surface or the irregularity and these positions are relatively displaced in accordance with the height or depth of the irregularity on the surface. This embodiment provides a further application of the invention: examination of a surface for damaging irregularities. Specifically, the surface may be a road surface. The light source and imaging means may be located on the underside of a road vehicle, the light source being arranged to project light in at least one light sheet onto the road surface and the imaging means being arranged to reveal irregularities in the road surface as deflections in an image. Thus roads can be checked with a view to repair before excessive damage is caused.
The light source preferably comprises solid state photoemitters arranged to emit infrared radiation of wavelength less than 1 xcexcm. This provides advantages in security, convenience and accuracy. The projected light will not be visible to the naked eye and so avoids the distraction of permanent lighting. In security systems, this also reduces the likelihood of evasive measures being taken to avoid detection. Furthermore, the system relies on good signal to noise from the projected light striking the body beneath. This requires strong contrast and minimising the effects of ambient lighting is therefore important. Typically, the ambient lighting of buildings does not contain much infrared intensity in the sub-1 xcexcm wavelength range.
A detection system arranged in a body counting implementation may be combined with a recognition system and incorporated into a second detection system. The recognition system is arranged to respond to the presence of predetermined individuals, preferably by detection of a radio-frequency marker tag, and thereby to enable the second detection system either to associate each count recorded by the body-counting detection system with a member of the set of predetermined individuals or to register an intrusion. This system provides security without being obtrusive. It can be operated with a default unlocked door or gate making it very attractive for fast throughput of personnel.
In another aspect, the invention provides a method of detecting objects travelling on a surface comprising the steps of:
(a) projecting at least one sheet of light onto the surface,
(b) selectively imaging the intersection of each sheet of light with the surface through imaging means whose optical axis is substantially parallel to the projection direction and offset from the projected light, and
(c) detecting deflections of imaged intersections of each projected light sheet.
This invention exploits the effects of perspective when viewing from a particular direction. An object interrupting a light sheet can be readily detected as an outward movement of an imaged intersection in the field of view. It thus provides for non-intrusive detection of movement through the system.
The method may also comprise the steps of:
(a) associating each deflection of an imaged intersection with the passage of an object through the projected light sheet responsible for said imaged intersection,
(b) responding to each deflection in accordance with the deflection being caused by the passage of an object through the associated light sheet.
This provides for an appropriate system response to be made to a deflection of an imaged intersection. This response may vary from recording a count to initiating a complex signal processing routine. It therefore confers a utility advantage in increasing the adaptability of the method of the invention to different purposes.
The method may further comprise the steps of:
(a) associating a deflection of one imaged intersection with a subsequent deflection of the other imaged intersection, and
(b) counting the number of deflection pairs.
This enables the direction of travel to be determined from the order in which the light sheets are disturbed and a count of interruptions to be kept. This provides the system with the capability for constantly monitoring the population within a designated area.
In another aspect of the invention, a method of detecting surface irregularities comprises the steps of:
(a) projecting at least one sheet of light onto a surface,
(b) selectively imaging the intersection of each sheet of light with the surface with imaging means whose optical axis is substantially parallel to the projection direction and offset from the projected light sheet,
(c) moving each light sheet and imaging means relative to the surface, and
(d) detecting deflections of imaged intersections between each light sheet and the surface.
This aspect exploits the effects of viewing objects in perspective in order to monitor variations in surface height. It is capable not only of merely detecting irregularities but also of determining their structure. Surface size is not a critical issue, the system is able to detect a variation in height down to a limiting relative change in projection distance. Microscopic surface irregularities can be detected although projected light with correspondingly small dimensions will experience greater diffraction and related microscopic effects. Furthermore there is a fundamental limitation to the smallest spot size achievable. Allowance will have to be made for this in a microscopic implementation.
In order that the invention might be more fully understood, embodiments thereof will now be described with reference to the accompanying drawings in which: